A silicone adhesive layer has excellent heat resistance, cold resistance, weather resistance, electrical insulating properties and chemical resistance. Therefore, adhesive tapes and adhesive labels having a silicone adhesive layer are used under severe environments in which quality of the other adhesive agents such as an acryl type adhesive agent, an elastomer type adhesive agent, an urethane type adhesive agent and an epoxy type adhesive agent deteriorates.
One of the severe environments is a high temperature of 250 degrees C. or higher. For instance, this environment is used for masking or temporarily fixing semiconductor parts in a reflow process or an encapsulating process with a resin of the semiconductor parts. In recent years, semiconductor parts are heated and treated at a higher temperature and, therefore, it is needed to increase heat resistance of the silicone adhesive agent.
A lead-free solder alloy has been used in a reflow process for molding electronic parts. For instance, a reflow temperature in the recent manners is higher than that in the conventional manners and a peak temperature reaches sometimes to 280 degrees C. in a reflow oven. It is needed that an adhesive agent does not peel from a substrate during the reflow soldering and an adhesive tape is released from the substrate without remaining any stain on the substrate after the reflow soldering. In recent years, the reflow process sometimes needs a higher temperature such as 300 degrees C.
However, when an adhesive tape with a conventional silicone adhesive agent is released from a substrate after exposed to a temperature of 200 to 250 degrees C., the adhesive agent remains sometimes partly or wholly on the substrate. In the present specification, this is referred to as “adhesive residue”.
Japanese Patent No. 4619486 describes that an antioxidant is added in a siloxane binder for an adhesive tape in order to prevent the adhesive residue.
Japanese Patent No. 4180353 describes that a phenol antioxidant is added in an addition-curable adhesive siloxane composition. An adhesive tape having the adhesive siloxane composition is applied on a metal substrate such as copper one and released from the substrate with no adhesive residue, even after heated at 150 to 250 degrees C.
However, when this silicone adhesive agent is exposed to a higher temperature such as 270 degrees C. or higher, the antioxidant deteriorates and the effect of preventing adhesive residue is not obtained. Further, when a substrate is made of a metal such as copper, copper alloy and iron, the surface of the metal is oxidized to thereby bonds to the silicone adhesive agent strongly and, thereby, an adhesive remains on the substrate.
If a too much amount of the antioxidant is contained in the silicone adhesive composition, the silicone adhesive composition is turbid. A curing reaction is obstructed in a case where the silicone adhesive composition is additive curable. Further, an adhesion strength of the silicone adhesive composition on heating is lower, compared to a silicone adhesive composition containing no antioxidant.
Japanese Patent No. 5032767 describes an adhesive tape wherein an amount of low molecular weight siloxane in a silicone adhesive composition is decreased. The silicone adhesive composition comprises an addition reaction product (A) obtained by reacting an organopolysiloxane (a) which has alkenyl groups at the both terminals, a viscosity of 10,500 to 60,000 mPa·s, and is accompanied with a decreased amount of low molecular weight siloxanes and a diorganohydrogenpolysiloxane (b) having hydrogen atoms each bonded to a silicone atom at the both terminals. This Patent Publication describes that the amount of low molecular weight siloxanes is decreased by thin-layer stripping. However, when the viscosity of the organopolysiloxane is larger than the aforesaid upper limit, it is difficult to decrease the amount of low molecular weight siloxanes by thin-layer stripping. Further, the main chain of the addition reaction product (A) in the silicone adhesive composition is —O—Si—CH2—CH2—Si—O—, but not —O—Si—O—Si—O— and, thus, the addition reaction product (A) has many carbon-carbon bonds, so that its heat resistance is poor. The viscosity of component (a) is smaller than that of an organopolysiloxane used in the usual silicone adhesive agent, so that an adhesion strength of the adhesive tape is sometimes poor. Further, the addition reaction product (A) is prepared by the reaction in the presence of a catalyst which may remain in the addition reaction product (A) to cause discoloration, so that transparency of the composition is sometimes poor.